Systematic- and proteomic-wide studies of the biological functions of proteins have drawn increasing interest during the past several years as the realization that mRNA expression levels may not accurately reflect the activity/expression level of their corresponding protein. And proteins, not genes, are the true targets of modern medicines. The prevailing approach for analyzing protein function in vivo is use of cell-based assays. These types of assays are used to study the function of one particular gene in a cellular context by gene transfection and protein delivery.
Protein delivery, which is known in the art as protein transduction, is the process by which a peptide or protein motif is delivered across the plasma membrane into the cell. Traditionally, methods to introduce antibodies, peptides or other membrane-impermeable molecules into cells include micro-injection and electroporation. The obvious disadvantages of these techniques are that they tend to be toxic to the recipient cells, they are non-specific (i.e., anything can enter or exit the cell once the membrane is disrupted), and they exhibit low transduction efficiency and substantial variability. To overcome the disadvantage associated with these techniques, researchers have developed a number of protein-transduction domains (PTDs) that mediate protein delivery into cells. These PTDs or signal peptide sequences are naturally occurring polypeptides of 15 to 30 amino acids, which normally mediate protein secretion in cells. They are composed of a positively charged amino terminus, a central hydrophobic core and a carboxyl-terminal cleavage site recognized by a signal peptidase. Recently, researchers have shown that a number of membrane-translocating peptides can successfully mediate delivery of polypeptides, protein domains, and full-length protein, including antibodies into cells using solution-based protein transduction protocols. Recently, researchers have also demonstrated the use of lipid liposomes or the like for protein delivery. Traditionally, however, these approaches have been limited since they are solution-based formats. Only one gene or protein may be studied per assay. As there are more than 35,000 genes present in the human genome, and approximately 10,000 of these genes are expressed as proteins in any given cell type, a high-throughput method for studying gene function is needed. It would be desirable to provide systems, including, but not limited to, methods, microparticles, and kits for multiplexed analysis protein and cell interaction within a single experiment.